Late quaternary climate variability in Southern Ocean Atlantic sector [Elektronische Ressource] / vorgelegt von Wenshen Xiao

Wenshen

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2011
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Late Quaternary climate variability in Southern Ocean
Atlantic Sector



Kumulative Dissertation
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
– Dr. rer. nat. –


Am Fachbereich Geowissenschaften
Der Universität Bremen


vorgelegt von


Wenshen Xiao

Bremen, 2011



This PhD thesis was conducted within the Marine Geology Group of the
Alfred-Wegener-Institut für Polar- und Meeresforchung (AWI), Columbus Strasse, 27568
Bremerhaven, Germany, between February 2007 and March 2011. The study was embedded
in the topic Past polar climate and inter-hemispheric coupling at the AWI. This study was
supported by Deutscher Akademischer Austausch Dienst (DAAD) for 3 years, and further
supported by AWI.






Gutachter der Dissertation:
Prof. Dr. Helmut Willems
Prof. Dr. Bernhard Diekmann

Weitere Mitglieder des Prüfungsausschusses:
Prof. Dr. Ruediger Henrich
Dr. Rainer Gersonde
Dr. Mariem Saavedra-Pellitero
Caroline Clotten

Tag des öffentlichen Kolloquiums:
23. May 2011
Geo-Hörsaal, 16 Uhr c.t.

Abstract I
Abstract

Climate variability of the late Quaternary, especially the Last Glacial (LG) to the Holocene, has
become the most heated topic for the recent decades, which helps to better understand the shape of
current and future climate on our planet. The long term glacial/interglacial changes have been
associated to insolation changes controlled by earth’s orbit, whereas the millennial scale variations are
mostly accepted to be modulated by the “bipolar seesaw” mechanism which redistributes heat between
the northern and southern hemispheres through the Atlantic Meridional Overturning Circulation
(AMOC). The validation of such hypothesis is hampered by the very limited high resolution records
from the high latitudes Southern Ocean.
Situated at the southern end of the AMOC, Southern Ocean Atlantic sector represents one of the
key regions for understanding the global climate change. The warm and cold water routes (WWR,
south of Africa and CWR, Drake Passage/Scotia Sea) connect the South Atlantic to South Indian and
Pacific Oceans, respectively; and the Weddell Gyre connects the open ocean South Atlantic to the
Western Antarctic Shelf Ice (WASI), where nowadays the cold surface water and Antarctic Bottom
Water (AABW) are generated beneath. These water masses represent the most important constituents
of the AMOC in the Southern Hemisphere. This PhD project generated a series of new diatom based
high resolution marine records covering wide area of the high latitudes South Atlantic, including from
the Bouvet Island area and the Scotia Sea, aimed to provide new insights of the response and drive in
Southern Ocean in the context of late Quaternary global climate change.
With focusing on the LG to Holocene time period, by integration of our new generated and other
existing records from the Southern Ocean Atlantic and Western Indian sectors, a detailed regional age
14model for the past 30 kyrs is established by AMS C dating and regional core correlation, which can
be a template for further paleoenvironment reconstructions in this area. Our reconstructions suggest
2-3 C cooling in the LG south of the modern Polar Front compared to modern conditions. Winter sea
ice in the Bouvet Island area expanded by 5 latitude, the more expanded sea ice field resulted in the
stronger tropical Atlantic cooling than other tropical oceans by the reduction of warm water to the
South Atlantic via the WWR, and intensive export of carbon to the deep Southern Ocean. The two
steps of deglacial warming are mainly concurrent with the Heinrich stadial 1 and Younger Dryas
cooling in the northern hemisphere which support the bipolar seesaw from the marine archives. The
North Atlantic Deep Water (NADW) provided the second source of warming from below to the high
latitudes South Atlantic which extended the first step warming till 14 cal. ka (kyr BP). The temporal
cooling at ca. 14-12 cal. ka is possibly caused by the melt water input from the Antarctica. Our records
support the Southern Ocean control of the deglacial atmospheric CO rise. The early Holocene 2
optimum marks the warmest period and the strongest cold water reduction and confined at ca. 11-10
cal. ka in the southern cores in the study area. Sea ice probably retreated south of modern conditions
and maximum opal deposition southward expanded to at least 55 S. The mid-late Holocene cooling in
the study area may be related to the cold water expansion from the Weddell Gyre with the developing
cavity under the WASI. The Holocene climate development may have also interplayed with the
NADW which represents a rapid resumption at the early Holocene and slight decline during the
mid-late Holocene.
The high resolution Holocene record from the central Scotia Sea suggests a stable climate at the
core site. The early Holocene high productivity is maintained by the enhanced upwelling at that time.
The Holocene reservoir change at the core site is evidenced, which is in agreement with the Southern II Abstract

Ocean ventilation history. The pre-Holocene release of CO from the Southern Ocean strongly 2
14lowered the C which caused the low surface ocean reservoir at the early Holocene at the core site. atm
The centennial scale climate variability may be mainly linked to solar activity and also influenced by
the sea ice induced freshwater variability.
Stratigraphis of long term records from the Scotia Sea covering the past 300 kyrs were established
by a combination of radiocarbon chronology, correlation of magnetic susceptibility (MS) to Antarctic
ice core dust/climate records, diatom biofluctuation stratigraphy, and geomagnetic chronology. Good
consistency of the age models by these proxies improves the reliability of our stratigraphies and
indicates the applicability of these approaches. However, detailed investigation show that, the
radiocarbon chronology can be affected by the changes in carbon reservoir and fossil carbon
contamination in the study area; the abundance pattern of diatom species Eucampia antarctica can be
used to identify the past 6 marine isotope stages (MIS) while the fluctuation weakens during MIS 7
and 8; the reliability of geomagnetic chronology weakens at low sedimentation rates conditions. The
MS-Antarctic ice core correlations represent high efficiency and the best reliability due to the high
sensitivity to the changes of surrounding source regions and current systems, which is closely related
to the climate changes in the Southern Ocean. In addition, a possible correlation between the ash
layers found in our Scotia Sea cores and the Antarctic ice cores is established, which can be used as
additional age markers for further studies in this area.
Zusammenfassung III

Zusammenfassung

Die Klimavariabilität im Spätquartär, speziell während des letzten Glazials (LG) und des Holozäns,
ist eines der heißesten Themenfelder der letzten Jahrzehnte, welche zum Verständnis der Ausprägung
des momentanen und zukünftigen Klimas unseres Planeten beitragen. Langzeitige
Glazial/Interglazial-Änderungen wurden in Verbindung gebracht zu Änderungen in der Einstrahlung,
kontrolliert durch den Erdorbit, wobei Variationen auf der Jahrtausendskala weitestgehend akzeptiert
durch den Mechanismus der “bipolaren Wippe” moduliert werden, welcher für einen Wärmeaustausch
zwischen Nord- und Südhemisphäre über die “Atlantische Meridionale Umwälz-Zirkulation” (AMOC)
sorgt. Die Bestätigung solch einer Hypothese wird behindert durch sehr beschränkt zur Verfügung
stehende hochauflösende Archive der hohen Breiten des Südozeans.
Am südlichen Ende der AMOC gelegen repräsentiert der Südatlantiksektor eine der
Schlüsselregionen zum Verständnis globaler Klimaänderungen. Die Warm- und Kaltwasserrouten
(WWR, südlich Afrika und CWR, Drake Passage/Scotia Meer) verbinden den Südatlantik mit dem
Indischen und Pazifischen Ozean; die Weddellgyre verbindet den offenen Ozean des Südatlantiks mit
Westantarktischen Eisschelf (WAIS), der Bildungsregion kalten Oberflächenwassers sowie
Antarktisches Bodenwasser (AABW). Diese Wassermassen stellen die wichtigsten Komponenten der
AMOC auf der Südhemisphäre dar. Dieses Promotionsarbeit generierte eine Serie neuer,
hochauflösender mariner Aufzeichnungen basierend auf Diatomeen, welche ein weites Gebiet der
hohen Breiten des Südatlantiks abdecken, inklusive des Gebietes um Bouvet Island und dem Scotia
Meer, und zielt darauf ab, neue Einblicke in Reaktion und Antrieb des Südozeans im Kontext
spätquartärer globaler Klimaänderungen zu liefern.
Mit Fokus auf die LG bis Holozän-Periode wurde ein detailliertes regionales Altersmodell für die
letzten 30 ka durch Integration unserer neu generierten und existierenden Aufzeichnungen der
14atlantischen und westindischen Sektoren des Südozeans mittels AMS C Datierung und regionaler
Kernkorrelationen errichtet, welches als Grundlage